Automatic volume control for radio receiving apparatus



March 8, 1938. H. DIAMOND ET AL AUTOMATIC VOLUME CONTROL FOR RADIORECEIVING APPARATUS 2 Sheets-sheet 1 Filed Dec. 27, 1955 March 8, 1938.H. DIAMOND Er AL AUTOMATIC VOLUME CONTROL FOR RADIO RECEIVING APPARATUSFiled Dec. 27, 1935 2 Sheets-Sheet 2 Patented Mar. 8, 1938 limitiAUTOMATIC VOLUTVIE CONTROL FR RADIO RECEIVING APPARATUS Harry Diamondand Francis W. Dunmore, Washington, D. C., assignors to the Governmentof the United States, represented by the Secretary of CommerceApplication December 27, 1935, Serial No. 56,348

3 Claims. (CI. Z50- 11) (Granted under the act of March 3, 1883, as

amended April 30, 1928; 370 (B. Gr. 757) The invention described herein,if patented, may be manufactured and used by or for the Government forgovernmental purposes without the payment to them of any royaltythereon.

'Ihis invention relates to improvements in automatic volume control forradio receiving apparatus. The primary object is to provide an automaticvolume control for a radio receiver in which the volume level isinfluenced only by the desired signal and not by an interfering signal01 static. The invention is applicable where fixed modulationfrequencies are used at the transmitter, such for example as 65 cyclesand 862/3 cycles used at radio range beacons. In one form of ourinvention reeds tuned to these frequencies operate in the output of theradio receiving set, the output of the reeds (which is purely 65 and 86%cycles with all interferencel and static eliminated) is amplified andrectified and applied as a negative bias to the radio frequency tube ofthe receiving set, thus giving automatic volume control.

Prior to our invention it was customary to allov'v either some of therectified carrier or the audio-frequency output to return to the gridsof the radio frequency tubes as a negative bias, in order to give theautomatic volume control feature. Thus any carrier or audio frequencywhich passed through the receiving set reduced its sensitivity andconsequently the desired signal output. With this invention the setsensitivity is controlled only by the particular audio note or noteswhich it is desired to receive-for instance, assume a pilot desires toreceive a radio beacon signal consisting of a carrier modulated at 65cycles and 862/3 cycles, and Wishes to have the output signal level tothe vibrating reeds held automatically at a substantially constant levelas the input signal to the receiver is increased, due to the approach ofthe airplane to the beacon station. With our invention the output of thevibrating reeds serves to give the negative grid bias, and, since thereeds are selective only to the frequencies it is desired to receive,only these frequencies have an eiect in controlling thesensitivity ofthe receiver. In this way an interfering note or crash of static doesnot affect the sensitivity of the receiver (unless it overloads it), andtherefore, the reed amplitude is not altered. With the old systeminterference would act to lower the sensitivity of the receiving set andconsequently decrease the desired signal output.

In another form of our invention which will be illustrated anddescribed, reed vibration is used to hold a substantially constantsignal output from the receiver by the method utilizing the amplitude ofreed vibration to tune or detune one or more of the r. f. stages in thereceiver. Other uses for and advantages gained by the use of thisinvention will be more clearly understood by reference to the followingdescription and diagrammatic drawings.

In the drawings- Figure 1 shows an application of the invention to aradio receiver for a radio beacon with carrier modulated at two radiofrequencies,

Figure 2 shows another form of the invention.

In Figure 1 the output of the reed converter, which output operates thevolume indicator, is ampliiied and rectified and applied to the grids ofthe radio frequency tubes in the receiving set to 'give automatic volumecontrol. In this iigure the numeral l designates the receiver of thesignals of the radio beacon. This includes radio frequency amplifyingtubes 2 and 3; detector tube #i and audio amplifier 5. The outputterminals of the receiver are indicated by the nu-` meral S which may beconnected to a lovv pass filter 5a, the purpose of which is to providean additional safeguard against frequencies higher than the beaconmodulation frequencies of 65 cycles and 86% cycles from passing throughthe reed converters, presently to be described.

The numeral 'l is the terminal of the receiving set through which gridsof tubes 2 and 3 receive their bias; 3 a double throw single pole switchwith moving contact connected to terminal l.; 9 is one iixed contactgrounded, and it) another fixed contact, the latter being connected tothe minus end of a suitable biasing resistor in the circuit, thepurposes of the parts just mentioned being the subject of presentlyfollowing description.

The numeral ii designates a reed converter which consists of the 65cycle reed i2 with which is associated the driving coil i4 and an 862/3cycle reed i3 with which is associated the driving coil i5. Thesedriving coils are connected to the output filter 8a. or directly to theoutput terminals 6. An output coil Iii is arranged to be excited by thereed i2, and H is a similar coil excited by the reed i3. The A, C.output of coil l5 is rectified by oxide rectifier i8, and the similaroutput of coil Vi is rectified by rectifier I9. The pulsating D. C.output from rectiiers i3 and i3 is connected in aiding polarities to theterminals of a resistance 2t. The common connection ltd from therectifiers to the center tap conection wb of the resistor is through thecourse indicator coil 22 which operates a needle of combined instrument32. This needle point gives a lateral guidance to the pilot and isoperated by the difference in the voltages delivered by the rectiflersI8 and I9. The voltage drop across the resistor 2l! is produced by thesum of the voltages delivered by the rectiiers. Numeral 24 designatesthe signal volume indicator coil of the instrument 32, to which thisvoltage drop is applied. The coil 24 operates the pointer 25 which isarranged to indicate the strength of the beacon signal delivered by thereceiver. The primary of an audio frequency transformer 28 is in serieswith the volume indicator coil 24 in the instrument.

A high capacitance electrolytic condenser 23 acts as a damping for thepointer 25, and as a shunt for the variable portion of the pulsating D.C. Voltage delivered by the resistor 2G, and the condenser 2| acts asdamping for the pointer 25.

Transformer 28 has an output voltage on its secondary because itsprimary current is of a pulsating nature and the output from thesecondary of the transformer is therefore proportional to the outputfrom the 65 cycle and 86% cycle reeds in the converter II and to nothingelse, and it is from this output that the control voltage is derived forholding the output signal level constant as will appear. A signal leveladjustment resistor is shown at 2l which is set at a volume to give thedesired signal level at which the automatic volume control is to holdthe output of the beacon receiver. A condenser 29 in shunt with thesecondary of the transformer 2B may be used when a low pass filter isnot used in the receiving set output. The purpose of the condenser 29 isto shunt any frequencies higher than the 65 cycle and 86273 cycle whichmay have passed through the reed converter, due to magnetic couplingbetween coils I4 and I5, and I5 and I'I.

A type 75 electron tube is shown at 33 which may be of the duo diodetriode type with the two diodes connected together, as indicated atforming the rectifying elements for the output oi' the triode portion ofthe tube 33. The control grid 34 of tube 33 is connected to the outputof the secondary of the transformer 28. The numeral 3'! designates thecathode, and 36 is the plate of the tube 33. A resistor 3! is providedto give the grid 34 the proper negative bias and across the resistor 3iis a by-pass condenser 30.

The output from the plate 38 passes through primary of transformer 4I.11D is a high frequency by-pass condenser serving a function similar tocondenser 25, and 38 is a usual radio frequency by-pass condenser tocathode. Conductor 39a goes to the D. C. plate supply source andconductor 391) goes to the positive side of the lament supply source,and 39 designates a filament resister. The output of secondary oftransformer 4I is rectified by the diode elements of tube 33 giving D.C. voltage drop across the resistance 42. The positive end of theresistor 42 is grounded and the negative end is connected throughresistor 13 and switch 8 to terminal 'i of receiving d set I, which inturn is connected to the grids of r. f. tubes 2 and 3. The connection ofresistor 52 to terminal 'i is made through double throw single poleswitch B when the movable contact of switch 8 is on terminal i5. Withswitch 8, therefore, on contact i5, the grids of radio amplifying tubesand 3 receive a negative bias from the resistor i2 which varies inamount depending upon the strength of the signal delivered to set I. Thestronger the signal delivered to set i, the greater will be the negativebias delivered to tubes 2 and 3, and consequently their sensitivity willbe reduced to such that the signal output of set I will be heldsubstantially constant. With switch B on contact 9, the grids of tubes 2and 3 are grounded and the automatic volume control feature isinoperative; the set being then adaptable for manual volume control.Referring now to Figure 2, there is here shown another form of ourinvention in which the reeds operate in a different way to give theautomatic volume control feature; that is to say, that instead ofproducing a negative bias the reeds tune and detune one or more of theradio frequency stages of the receiving set to give automatic volumecontrol. In this figure, 45 represents the usual radio beacon receiverwith input terminals 58 connected to the airplanes antenna 154. :i5 isthe primary of the input coupling coil. il is the secondary of said coilwhich latter is tuned to the desired beacon signal by means of thevariable condenser 58, and in shunt with this condenser are two parallelcondensers whose capacitance varies with the amplitude of vibration ofthe 65-cycle reed 56 and the 862/-,-cycle reed 56. Plates 52 and 53 withreed 50 form one condenser in shunt with 58, and plates 54 and 55 withreed 56|, form the other condenser in shunt with 58.

Reed 55 is driven by driving coil 5I and reed 56 is driven by drivingcoil 51. These coils are connected to the output terminals 69 of thereceiver. 59 is a radio frequency amplifying tube with its gridconnected to condenser 58 and its plate to primary of coil 55. Secondaryof coil 6I is shunted by tuning condenser 62 connected to the grid ofdetector tube 63. 54-55 is the usual audio frequency couplingtransformer between detector tube 53 and audio frequency amplifying tubeE5. The output of tube 66 passes through output transformer tri- 68 tooutput terminals 5S, which in turn may be connected to the driving coils'I2 and "i3 of the tuned reed course indicator 15 having a (i5-cyclereed TI and an 86%-cycle reed it. This arrangement functions as follows:The beacon signal is tuned in with condensers 62 and 58. 58 is thentuned (by adding capacitance) until the desired amplitude of vibration(about 8 mm.) is obtained on reeds 'il and "I4, as well as on reeds 55and 55. Now, if the signal level delivered to secondary 4l increases,this increase will go through the set and come back to driving coils 5iand 5'! and the reeds actuated by these coils will increase inamplitude, thus increasing the capacitance from condenser plates 52-53to reed and from condenser plates 54 and 55 to reed 55. This increasedcapacitance being in shunt with condenser 58 will tend to further detunethis condenser and thus reduce the signal to normal again.

We claim:

l. A radio receiver comprising a radio frequency amplifying means, adetector and audiofrequency amplifying means, two mechanicallytunedselective means coupling said audio amplifying means, a rectiiying meanscoupling each mechanically-tuned selective means, indication means forindicating the diierence in the pulsating direct current output of saidtwo rectifying means and additional indicating means for indicating thesum of the pulsating direct current outputs of said two rectifyingmeans, means for changing the sum of the pulsating direct currentoutputs of said rectifying means into alternating currents, means foramplifying said alternating currents, a third rectifyng means forrectifying and ltering said alternating currents and applying theresultant direct current to said radio frequency amplifying means insuch man,- ner that the sensitivity of said radio frequency amplifyingmeans is controlled by the amplified sum of the outputs of the said rsttwo rectifying means.

2. A radio receiver with electron tubes having grid filament and plateelectrodes, said receiver having a plurality of output circuits eachconsisting of a tuned reed with coil actuating means each of said reedsbeing mechanically tuned to substantially a different frequency, coilgenerating means associated with each of said reeds for derivingalternating current of constant frequency from the vibration of saidreeds, means for rectifying said derived alternating current and meansfor impressing said rectified alternating current onto the gridelectrodes of certain of said electron tubes for controlling thesensitivity of said radio receiver.

3. A radio beacon course indicator comprising a radio receiver withinput and output circuits, two tuned reed lters each tuned to adifferent frequency With input and outputcircuits,

said input circuits being connected to the output of said receiver,rectifying means, with input and series connected output circuits, theinput of each rectifying means being connected to a different one of theoutput circuits of said tuned reed filters, a resistor, the pulsatingdirect current output of each rectifying means being connected inopposition to the terminals of said resistor, a dual indicatorinstrument with two coils and associated pointers, a transformer, one ofthe coils of said instrument being connected across said resistorthrough the primary of said transformer, the other coil being connectedto the center of said resistor and the series connection between the tworectiers, amplifying means connected to the` secondary of saidtransformer, and rectifying and filtering means connected to the outputof said amplifying means, the output of said filtering means beingconnected to said radio receiver in such manner that the output of saidfiltering means controls the sensitivity of said radio receiver.

HARRY DIAMOND. FRANCIS W. DUNMORE.

